Update Sync error pixels

[ayoy]

Task/Issue URL: https://app.asana.com/0/1201621708115095/1207559244852258/f

Description:
Rename count limit exceeded as object limit exceeded and add "Too Many Requests" pixels.

Steps to test this PR:
Since this change doesn't modify any logic related to processing errors, it's enough that you verify that pixel names are correct. The simplest is to add this snippet of code at the bottom of AppDelegate's applicationDidFinishLaunching:

PixelKit.fire(GeneralPixel.syncBookmarksObjectLimitExceededDaily, frequency: .legacyDaily)
PixelKit.fire(GeneralPixel.syncCredentialsObjectLimitExceededDaily, frequency: .legacyDaily)
PixelKit.fire(GeneralPixel.syncBookmarksRequestSizeLimitExceededDaily, frequency: .legacyDaily)
PixelKit.fire(GeneralPixel.syncCredentialsRequestSizeLimitExceededDaily, frequency: .legacyDaily)
PixelKit.fire(GeneralPixel.syncBookmarksTooManyRequestsDaily, frequency: .legacyDaily)
PixelKit.fire(GeneralPixel.syncCredentialsTooManyRequestsDaily, frequency: .legacyDaily)
PixelKit.fire(GeneralPixel.syncSettingsTooManyRequestsDaily, frequency: .legacyDaily)

Then run the app and check the log. Compare output with what's expected in the Asana task.

Definition of Done:

• Does this PR satisfy our Definition of Done?

---

Internal references:

Pull Request Review Checklist
Software Engineering Expectations
Technical Design Template
Pull Request Documentation

[SabrinaTardio]

LGTM!